Regular Naruto and Sakura fanfictions usually focus on their positive aspects like friendship, love, and their growth as ninjas. But 'dark' ones might explore more negative emotions, darker storylines. And as for 'lemon', it's inappropriate content which regular ones don't have.
In some fanfictions, Naruto might move on from Sakura by focusing on his ninja duties. He throws himself into more difficult missions, realizing that his life has a greater purpose than just pining over Sakura. This way, he gradually distances himself from his old feelings as he becomes more dedicated to protecting the village.
Yes, it can be a somewhat popular trope in the Naruto fanfiction world. Some fans like to explore alternative relationship dynamics. They might be interested in seeing how Naruto's relationship with Sakura could develop in a scenario where he 'steals' her from Sasuke. It adds an element of drama and can lead to interesting character development as Naruto might have to overcome his own feelings, Sakura's prior attachment to Sasuke, and Sasuke's reaction if such a thing were to hypothetically happen.
Well, in the original, Naruto has a more light - hearted and positive pursuit of Sakura. But in the case of Dark Naruto, it would be more complex. His darker nature might make Sakura more hesitant at first. So their relationship would develop more slowly, with more internal struggles for Sakura as she tries to balance her feelings with her perception of his darker side.
Maybe Naruto travels the world to learn new ninja techniques. Along the way, he meets new people and forms new bonds, leaving his feelings for Sakura behind. Another plotline could be that he devotes himself to rebuilding the Hidden Leaf Village and his love life takes a backseat.
One consequence could be that Sakura might feel neglected or confused. If Naruto has always been a part of her life and suddenly stays away, it would definitely have an impact on her emotions. She might start to question what she did wrong or if Naruto has changed in some way.
In fanfiction, it could be a way to create new plotlines. By having Naruto stay away from Sakura, new conflicts and story arcs can be introduced. For example, it might lead to a situation where Sakura has to re - evaluate her own feelings and actions towards Naruto, or it could create a rift between them that other characters have to help bridge in the story. Also, some writers may simply not be interested in the typical Naruto - Sakura relationship and choose this as a way to deviate from the norm.
Often in fanfiction, Naruto moves on from Sakura when he experiences a major event. Say, he nearly dies in a battle and has an epiphany. He realizes that life is short and he should focus on things that truly matter. His feelings for Sakura seem less important compared to his newfound understanding of life, and so he moves on.